1. Field of the Invention
This invention relates, in general, to devices for measuring characteristics of gases and, in particular, to a new and useful apparatus and method for measuring the dew point of a gas.
A method for measuring the dew point using a sensor and a tempering device having a heater and a cooler, in which a surface of the sensor which is exposed to the atmosphere to be examined is brought through an adjustable tempering by the tempering device to a temperature which leads to the formation of dew on the sensor surface and in response to the formation of the dew, a specific sensor property is changed and the tempering device is regulated to obtain a constant dew layer thickness on the sensor surface, with the sensor temperature detected by a temperature sensor being indicated as the dew point temperature and using the cooler to create a variable degree of cooling in order to draw from the sensor more heat than is required for reaching the dew point temperature, using the heater to provide an additional heating of the sensor surface causing it to reach the dew point temperature, and for purposes of calibration the heating power of the geshichen is raised so far that complete evaporation of the dew occurs, and, upon evaporation of the dew, the specific sensor property is measured and supplied as a calibration signal to the evaluating unit.
For measuring the dew point of a gas, dew point sensors are used, which are kept at such a temperature that formation of dew on the sensor surface occurs. In the process, a specific sensor property is determined, for example, the capacity of a capacitor due to the great dieelectric constant of water, changes as a function of the dew formation. For setting and regulating the sensor temperature to a constant dew layer thickness, a cooling device is used as a tempering device. The temperature of the sensor through the tempering device, at varying vapor contents in the atmosphere, is made to follow continuously the differing dew points at any given time. A temperature sensor measures the temperature of the sensor and diplays it as the dew point temperature (German Pat. No. 32 32 995.) By dividing the tempering device into a separate cooling device and a separate heating device--as described in application No. P 36 33 015--the cooling capacity can be so apportioned so that it draws from the sensor more heat than would be required for reaching the dew point to be expected. In addition, a heating device of lower heating capacity is responsible for a more sensitive and more readily controllable heat supply for achieving the dew point temperature to be determined, and--in this way permits compensating for fluctuations of the dew point rapidly and precisely.
In the course of longer-lasting operation, or following repeated turning on and turning off of equipment operating according to this method, it may happen that particles are deposited on the sensor surface or substances which were dissolved in the condensate and are deposited upon being heated or dried. Depending on the nature of the residues and the extent to which they effect the specific sensor property utilized for taking measurements (for example, its capacity or electrical conductivity) the zero point can gradually become displaced in the course of the measurements without it being noticed and can thus lead to falsification of the measuring results.